The terrifying ordeal of two French ­students murdered in London was relived for the first time at the Old Bailey ­ today when one of the men accused of their killings changed his evidence and confessed to helping his accomplice tie them up in a burglary that went tragically wrong.

Dano Sonnex, 23, admitted entering the property where, the court heard, his co-accused was already holding down one of the students, Laurent Bonomo, 23, and was pushing his head into a pillow. Sonnex said he was told to do the same to the other victim, Gabriel Ferez, 23, who struggled but did not fight back.

Both students were almost naked. Ferez had been sitting on a futon in his underpants and was "edgy" and shouting loudly in French. Bonomo, he said, kept talking in English about his girlfriend.

Sonnex and Nigel Farmer, 33, are accused of murdering the two students, who were in London on an exchange trip to work on a DNA project at Imperial College. The men blame each other for the killings, which left the students with 244 stab wounds, including injuries that caused haemorrhages to the brain, in what prosecutors described as "an orgy of bloodletting".

Sonnex said he had been outside the flat in New Cross, south-east London, keeping a lookout when Farmer called for his help. He said that when he went inside he was told by Farmer to tie up Ferez, which he did with a pair of tights.

After looking for things to steal, Sonnex said he left the flat with a number of items including a bank card for which Bonomo had given him the pin number. Sonnex left wearing Ferez's glasses "in order to ­disguise myself". On returning, ­Sonnex was given another card and number by Farmer, and when that was retained by the cash machine he went back to the flat, he claimed, to discover the two men had been stabbed to death.

On the eve of the killings, Sonnex said, the pair had been dealing and taking drugs when Farmer claimed he had been robbed at knifepoint in the toilets of a club. The burglaries, Sonnex said, were an attempt to make up the money that was stolen – something that had made Farmer "angry".

The fathers of both the dead students listened as Sonnex claimed he tripped over the body of Bonomo as he lay on the floor. "His face was bloody, I remember that," he said. "My friend [Farmer] was sitting at the end of the bed in between the bed and the drawers. He was hissing and hissing and shaking his head and hissing. He was saying the words 'this is naughty, this is naughty' – that's all he kept saying: 'this is naughty'."

Farmer admits that later that day he set fire to Bonomo's flat and was badly burned in the explosion. He denies burglary and murder.

Asked by his barrister, Philip Misner, why he had changed his evidence, Sonnex said: "The allegations that were made against me [by Farmer] and the way they have been told, I just thought the jury would not believe I didn't kill these people. It's better to be truthful, I would rather just tell the truth and get it over and done with." He accepted that he was part of a burglary and an enterprise to hold the students against their will and that he knew Farmer was going to set fire to the flat. He denied killing Bonomo and Ferez.